After being hospitalized with continuing COVID-19 symptoms, Boris Johnson has now been transferred to the ICU after his condition deteriorated. Downing Street has announced that Dominic Raab, Foreign Secretary and First Secretary of State, will deputize for him as needed, and this had led to some confusion over Raab’s precise status. Contrary to popular belief, he’s not quite Britain’s ‘leader-in-waiting.’
While the office of president of the United States has a clearly defined line of succession, there is nothing analogous for the British Prime Minister. While there have occasionally been ministers with the title of ‘Deputy Prime Minister’ (e.g., Nick Clegg during the Coalition government of 2010-2015), this is simply a courtesy title and it does not confer any right of succession to the premiership. The British constitution does not recognize the office of ‘Acting Prime Minister,’ either. Other ministers may act for the Prime Minister from time to time (e.g., when the Prime Minister is on vacation), but they do not formally occupy the premiership when they do so. Many of the Prime Minister’s duties derive from custom rather than statute, so it’s possible to have a relatively informal delegation of power in these circumstances.
If the office of Prime Minister were to suddenly fall vacant (whether due to resignation or death), the Queen would need to select a successor, though her choice would still need to command the confidence of the Commons. In the event of an urgent vacancy, she would likely be guided by the advice of the Cabinet, whether that is expressed formally or informally (the Cabinet in turn is likely to be influenced by the views of the wider parliamentary party). In practice, her choice would need to be confirmed as leader of the governing party in order to remain Prime Minister for long. But even if someone is appointed as a temporary measure pending a full leadership contest, they are not considered an ‘Acting Prime Minister;’ they are the Prime Minister until someone else takes their place.
Needless to say, a Prime Minister appointed in unexpected circumstances is still subject to a vote of no confidence by the House of Commons.
So ultimately, Raab can only succeed Johnson if the Cabinet supports him.